1. Field of the Invention
The invention relates generally to a power steering system and a control method of a power steering system. More specifically, the invention relates to a power steering system used in a lane-position maintenance assist apparatus that assists a vehicle in maintaining the proper lane-position, and a control method of a power steering system.
2. Description of the Related Art
For example, Japanese Patent No. 3185726 describes a lane-position maintenance assist apparatus that assists a vehicle in maintaining the proper lane-position. The lane-position maintenance assist apparatus initially calculates the lane-offset amount that is the lateral offset between the center line of the vehicle, extending in the longitudinal direction of the vehicle, from the center line of the cruising lane; the yaw angle which the center line of the cruising lane forms with the center line of the vehicle, etc. by processing the image of the cruising lane captured by, for example, a CCD camera. Based on the result of calculation, the lane-position maintenance assist apparatus calculates the steering torque for assisting the vehicle in maintaining the proper lane-position (hereinafter, sometimes referred to as the “steering torque for lane-position maintenance assist”). The lane-position maintenance assist apparatus assists the vehicle in maintaining the proper lane-position by driving an electric motor included in an electric power steering system based on the calculated steering torque.
However, the lane-position maintenance assist apparatus drives the electric power steering system independently of the steering operation performed by a driver, that is, the input of the driver. As a result, the driver may feel a sense of discomfort, and the steering feel may deteriorate.
For example, when the driver operates a driver's steering wheel during the lane-position maintenance assist control, if the direction in which the driver operates the driver's steering wheel is the same as the direction in which the steering torque for lane-position maintenance assist is applied, the driver's steering wheel is operated unexpectedly lightly, causing the driver to feel unexpectedly less resistance in the operation of the driver's steering wheel. On the other hand, if the direction in which the driver operates the driver's steering wheel is opposite to the direction in which the steering torque for proper lane-position maintenance assist is applied, the driver's steering wheel is operated unexpectedly heavily, causing the driver to feel unexpectedly high resistance in the operation of the driver's steering wheel. Also, if the steering torque for lane-position maintenance assist fluctuates, the driver's steering wheel vibrates due to the reaction force caused by such fluctuation.